In a well-known process to make open porous sintered bodies of glass, glass of definite grain size is heated in a mold to a temperature, which is close to the softening point of the glass. By softening the glass grains are sintered at the area of contact. A drawback of this procedure is that open pore volumes of more than 40% are not achievable with sufficient stability.
In a similar process to make open porous sintered bodies of glass ceramics, ground ceramic powders of definite grain size are sintered one to another. By such sintering ceramic powders, the open pore volume is also limited to at the most 40% with sufficient stability.
Porous glass ceramics are obtainable by a selective extraction of crystalline glass (German laid-open print No. DE 23 59 735). However, in this case, too, open pore volumes with high percentage cannot be obtained at warrantable costs.
To make open porous sintered bodies of glass, glass ceramics or ceramics a series of procedures is known which Proceed from an admixture of organic substances to the sinterable powder. In the course of the heating process the organic substances are burnt up and thereby cavities are formed. The sintering process has to be handled in such a way that the connections of these cavities and consequently the open porous state is kept lasting (German Pat. No. DE 31 03 751, German laid-open print No. DE 32 07 623, Japanese laid-open print No. J 61 158 841). Starting from a glass powder for the procedure, there is the danger that the connections of the cavities may sinter together to form sealed pores. Therefore, the sintering process of the vitreous material must be performed at relatively low temperatures so that the achievable mechanical stability is limited. A further drawback of this process consists in the fact that the abilitY to flow through the open porous sintered glass bodies is diminished by the narrow connections between the cavities.
Processes are known to make open porous sintered bodies with defined pore diameters wherein soluble inorganic salts are used as pore-forming material. After sintering the inorganic salts are leached from the mixed body whereby the open pore volume evolves and the pore diameters have a defined size.
The German patent specification No. DE 33 05 854 depicts a process to make open porous sintered vitreous bodies wherein a glass powder is sintered with a soluble substance. As the sintered structure consists of glass the material properties of the sintered bodies are limited to such properties that are achievable by the vitreous material (i.g. temperature constancy, thermal dilation, chemical resistance). Generally, the annealing point T.sub.g is cited as an upper limit of the temperature constancy of glass. In the course of sintering, a calculated ion exchange between the glass powder and an inorganic salt only shows a small influence upon the material properties of the vitreous body because, predominantly, ions of the same kind exchange. Therefore, the material properties of the open porous sintered vitreous bodies are only variable by calculated ion exchange within narrow limits.
The European patent specification No. 0 117 484 depicts a process to make porous sintered bodies with large open pore volumes and defined pore diameters. This process utilizes a pluverized glass powder and/or crystallized glass-ceramic powder being mixed with an easily soluble inorganic salt. The inorganic salt serves as a pore forming material and is leached out after sintering. Using an already crystallized glass-ceramic powder is a drawback, in that this powder has lost the sinterability of the vitreous state. Therefore, only by additional measures can sufficient mechanic stabilities be obtained. One possibility is the addition of a glass powder which causes the combining strength. By this method a number of material properties change for the worse as the properties of the glass phase which is present limit the properties of the combined arrangement. A sintering process without addition of glass can be obtained in case of partial dissolution of the crystalline glass-ceramic powder by ion exchange with the cations of the inorganic salt. However, this effect may be used only with few compositions of glass-ceramic powder and inorganic powder.
U.S. Pat. No. 2,209,163 depicts a process to make highly porous and sound absorbing sintered bodies of ceramics. As the sintering temperatures of the ceramic powders are higher than the melting point of the water soluble pore forming materials, an additional binding agent or fluxing agent must be admixed for sintering at low temperatures. By addition of binding or fluxing agents the material properties of the ceramics are limited.